106 CHEMICAL ENGINEERING
Net operating cost:
(15,031.88/120,000) $0.125 per tonócarriers with rigid buckets.
(13,4:15.63/120,000) $0.112 per tonócarriers with pivoted buckets.
Suctioa conveyors developed originally for handling ashes from the ashpits of boiler furnaces have proved convenient and efficient devices for handling other fine material. The conveying duct is a system of piping from ^vhich the air is exhausted by a high-speed fan at the head end of the conveyor, Intakes are situated at loading points and the material handled is swept through the conveying duct and deposited in a receiving hopper located below the exhaust fan by the rush of air through the open intake and through the piping to the exhaust fan. The fine material is carried virtually in suspension and there is little abrasive wear on the walls of the conveying pipe on straight stretches. At elbows, removable wearing plates are necessary to withstand the scouring action of the impinging material, but otherwise the deterioration of the conveying duct is not rapid.
The carrying capacity of a suction conveyor depends upon the diameter of the conveying duct (pipe) and the weight of the material handled. Closed intakes must be tight to avoid leakage of air which would slow down the rush of air and material swept in through feeding intakes and cause the material in suspension to be precipitated, A convenient equation for ascertaining the capacity of suction conveyors in tons per hour is given as Formula 29.
W = 0.038Wd2 Formula (29)
W = Capacity in tons per hour,
d = Diameter of conveyor duct (pipe) in inches, and zof == Weight of material handled in pounds per cubic foot. A peculiarity of suction conveyors is that once the necessary degree of vacuum has been created in the system, it requires practically no more power to operate a long conveyor than it does a short oneówithin reasonable limits. With reasonably tight intakes, the formula for horsepower is given as Formula (30) and it will he noted that the power requirement is quite independent of the load carried, being simply that required to maintain the suction necessary to handle the capacity load.
Horsepower = 0.55d2 Formula (30)
The chief items of cost in any suction conveyor system are the powerful exhaust fan required and the tank with its accessories in which the load handled is deposited and these essential appliances are just as costly for a short system as for a long one. The expense entailed for the conveyor duct, intakes, etc., varies, but for all practical purposes the cost of a complete system will be found to vary very nearly directly with its size, diameter of conveyor duct, or conveying capacity. The relative cost of a complete suction conveyor system is then, expressed by the equation :
RCSS = 143.7J2 Formula (31)
Fixed charges amount to 8.5 per cent of initial cost of equipment per year and a flat depreciation rate of 10 per cent per year will cover deterioration, necessary renewals, etc.. The labor charge can be overlooked in computing the net operating cost for a suction conveyor system is invariably an adjunct to other operations against which the labor of loading the conveyor, etc., is chargeable, while such little attention as the system may require in the way of inspection, etc., may be charged up to general plant upkeep. Incidental expenses for lubricants, etc., will average about a cent per horsepower consumed.